Brugia malayi worms are major causative agents of human lymphatic filariasis (more commonly known as elephantiasis), a neglected tropical disease that is the second leading cause of morbidity worldwide. The mass drug administration programs universally used to control this painful, disfiguring, and incapacitating disease are at risk of failure due to the potential for drug resistance. The goal of this project is to define the mechanisms that determine the interdependencies between the parasitic nematode B. malayi and its essential bacterial endosymbiont, Wolbachia, in order to discover novel means of treatment that can eliminate the adult worms. The biology of the nematode and its endosymbiont are known to be co-dependent, but the basis of this relationship has yet to be elucidated. Eliminating Wolbachia from the parasites using antibiotics affects molting, reproduction, and survival of the worms, indicating that the bacteria are crucial for the development of the parasite; thus, they represent an attractive target for control of the infections. We intend to study the interaction of the parasite and its endosymbiont at the molecular level using candidate Brugia-Wolbachia protein complexes and pathways that have been identified as major players in the endosymbiotic relationship, and test the role of microRNAs (miRNAs) in regulating interactions. This approach has the greatest potential to expand our knowledge of this biological system, which is currently poorly understood. We propose a unique application of new technologies, in combination with traditional techniques, to generate data that will answer important questions about the endosymbiotic relationship and, in so doing, provide groundbreaking information to the Wolbachia research field in general. Our three primary aims are to: 1) define the physiological role of Wolbachia surface proteins in the endosymbiotic relationship; 2) determine if miRNAs regulate B. malayi nuclear gene expression in the Wolbachia-Brugia relationship; and 3) characterize the essential functions provided by Wolbachia for B. malayi molting.